Polyvinyl alcohol (which may hereinafter be abbreviated as “PVA”) is known as a water-soluble synthetic polymer. PVAs are used as a material for vinylon, which is a synthetic fiber, and also used in a wide variety of applications such as in paper processing, fiber processing, adhesives, stabilizers for emulsion polymerization or suspension polymerization, binders for inorganic substances, and films.
PVA films as mentioned above have come to be used as water-soluble films for hermetically packaging a predefined amount of (for unit packaging of) any of various chemicals, such as agricultural chemicals, laundry detergents, bleaches, toiletry products, and industrial chemicals, to form a package. The package, when used, is put in water to dissolve or disperse the packaged product together with the packaging film in water. The unit packaging offers the following advantages, for example: the package can be used without direct contact with a hazardous chemical; the package can be used without the need for weighing since the amount of the packaged product is predefined; and the need for post-use disposal of a packaging container enclosing the chemical is eliminated.
PVAs with a higher degree of saponification have higher crystallinity and hence a higher content of crystalline portion insoluble in cold water. For this reason, in cold water-soluble films such as those for use in unit packaging, partially-saponified unmodified PVAs have been used instead of so-called “completely-saponified” PVAs which have a high degree of saponification. A water-soluble film produced using a partially-saponified unmodified PVA is characterized, for example, by being highly soluble in cold water and having high mechanical strength.
When any conventional partially-saponified unmodified PVA film is used to package an alkaline or acidic material, acetoxy groups remaining in the partially-saponified unmodified PVA film are saponified during storage, which unfortunately promotes crystallization and makes the film insoluble. Furthermore, when a film produced using a partially-saponified unmodified PVA is used to package a chlorine-containing compound such as an agricultural chemical or disinfectant and the resulting package is left for a long period of time, the film is stained or hardened and, at the same time, undergoes a decrease in solubility over time to become insoluble or poorly-soluble in water so that the chemical as packaged in the film disadvantageously becomes difficult to dissolve or disperse in water. In addition, a film produced using a partially-saponified unmodified PVA lacks sufficient stiffness because of low crystallinity and thus has poor processability.
To solve these problems, Patent Literature 1 discloses a water-soluble film formed from a PVA containing an oxyalkylene group, a sulfonic acid group, or a cationic group. Patent Literature 2 discloses a water-soluble film formed from a composition containing: a modified PVA having a monomer unit containing a carboxyl group and/or sulfonic acid group; and a polyol. Patent Literature 3 discloses a water-soluble film formed from a modified PVA containing a N-vinylamide monomer unit and a carboxyl group and/or lactone ring.
In recent years, a water-soluble film that meets all performance requirements as to cold water solubility, mechanical strength, and chemical resistance has been demanded, for example, in terms of workability and environmental protection. The conventional PVA-containing water-soluble films mentioned above cannot fully meet such performance requirements.